Air Doors Slammed Shut @ 1000rpm, 8V71

[Traveler 45C]

I think the kids were playing around with the shutdowns. The trip mechanism must have been right on the edge of letting go because the doors didn’t shut until I ran up the engine. Got to about 1000rpm, heard the air door slam shut and the engine died. Now I’m worried about the blower oil seals being compromised.

I’m aware of the potential for runaway should those oil seals leak. Should I pull the air inlet and inspect the blower seals as a precaution or am I worrying too much?

 

[bostonhatteras]

thats a dilemma! i assume you haven't run the engine since the air door clamped down and shut down the engine. but if you did run it was there any blue smoke, or how about an increased idle speed due to more 'fuel', or hunting of the idle speed? you must have thought about the options, take the parts apart and loose probabaly two weekends or take the risk running it. looking at the oil seal issue, and i dont have my factory manuals with me as i write about this, i wonder if they are full oil pressure fed journals or more a seal for mist and splash. if they are direct fed from the oil gallery then i think i would tear down the parts and check. if they are for more or less mist and splash with no more pressure on them than the internal crank case pressure i would try to run the engine. if they are compromised and its only for oil mist i would think you wouldnt have enough additional fuel for a run away considering you would naturally pull the throttles back all the way first. i also want to say im not a dd trained mechanic and my opinion on this is probabaly no more accurate that any other laymans opinion ie; i dont want to be responsible for someones engine! but this is how i would proceed and i have the same engines and boat. btw the damper design does suck. my personal experience with this is that my engines (both!) where shut down from idle by a helper about 10 years ago and we havent had any issues. he said 'it says shut down right on it', who can argue with that!
Russ

 

[MikeP]

DD recommends that the air doors be checked annually for proper operation, including running the engine at idle and shutting the doors to verify that the engine will shut down.

1000RPM isn't all that high, less than 500 RPM above idle. Frankly, I would just start the engine. Remember, you still have the air doors and they obviously work fine so there is no danger of a runaway, no matter how much oil gets past the seals...you just trip the air doors again.

IMHO, I doubt there is any damage at all to the seals and, once started, it will be easy to tell by the engine's reaction if there is any difference. If the idle speed is higher than it used to be, it would be an indicator of more fuel present.

 

[Traveler 45C]

The engine ran fine after, the idle seemed the same, but I didn’t check for blue smoke. I’ll do that today.

 

[krush]

So what's the problem again? Airdoors are there to be used, otherwise some enginerd would've never put them on the engine......

 

[Will]

No need to worry unless you see a lot more smoke or other symptoms. The blower drive gears, shafts, etc. are drip lubricated. Although such an event is not good practise, the chances of damage are minimal.
Will

 

[MikeP]

If the airdoors are tripped under heavy load, the air is shut off to the blower box but the engine is still trying to pull a lot of air. Therefore the vacuum applied tends to want to suck anything "into" the airbox. The blower rotor seals are the weakest link and that's where all the vacuum will be applied, possibly damaging the seals and, from that point on, allowing lubricating oil into the blower and then into the combustion chamber which is not accounted for by the governor. THe airdoors are supposed to be for emergency use.

That's the theory, anyway. I don't know if this actually WILL damage the seals but that's what folks who have been around these engines for a long time say. I check my doors periodically per DD while at idle. Last time I checked them one engine would NOT shut down though it dropped RPM and sputtered. I decided not to bother removing the airhorn and adjusting the door since I figured if it will barely idle, it will certainly stop a runaway.

 

[Brian Degulis]

The blowers on our engines are just like any other rotary blower. When you cut of the air supply completly the air in the blower gets trapped and this un loads the blower and stops the vacum. Next time your in the mens room put your hand over the hand dryer outlet. You will hear it speed up because the motor has un loaded.

You probably have nothing to worry about because it's only a moment of peak vacum when it first shuts. If you want to be sure then remove the air horns and screens. Clean all debris away from the blower then start the engine and carefully shine a light down on eack side where the blower shafts pass thru the housing. If the seal is bad you will see oil slinging out away from the shafts.

Again be carefull doing this the blower will eat just about anything including your fingers. I worked on an engine that the operator thought was running away while the blower was open. The guy paniced and grabed a pillow from a state room and pushed it into the blower. You would be amazed how much of the pillow it consumed.

Brian